1. Field of the Invention
The present invention is related to the field of devices emitting electromagnetic radiation. More in particular, a semiconductor device that emits radiation at a predetermined wavelength is disclosed. A method of producing such device and applications of such device are also disclosed.
2. Description of the Related Art
Semiconductor devices that can emit non-coherent or coherent electromagnetic radiation are known in the art. A number of publications on semiconductor based electromagnetic radiation emitters deals with Light Emitting Diodes (LEDs) or Microcavity LEDs or Microcavity Lasers or Vertical Cavity Surface Emitting Lasers. Examples of such publications are:
H. De Neve, J. Blondelle, R. Baets, P. Demecster, P. Van Daele, G. Borghs, IEEE Photon. Technol. Lett. 7 287 (1995);
E. F. Schubert, N. E. J. Hunt, R. J. Malik, M. Micovic, D. L. Miller, "Temperature and Modulation Characteristics of Resonant-Cavity Light-Emitting Diodes", Journal of Lightwave Technology, 14 (7), 1721-1729 (1996);
T. Yamauchi and Y. Arakawa, Enhanced and inhibited spontaneous emission in GaAs/AlGaAs vertical microcavity lasers with two kinds of quantum wells. Appl. Phys. Lett. 58 (21), 2339 (1991);
T. J. de Lyon, J. M. Woodall, D. T. McInturff, R. J. S. Bates, J. A. Kash, P. D. Kirchner, and F. Cardone, "Doping concentration dependence of radiance and optical modulation bandwidth in carbon-doped Ga.sub.0.1 In.sub.0.49 P/GaAs light-emitting diodes grown by gas source molecular beam epitaxy" Appl. Phys. Lett. 60 (3), 353-355 (1992);
D. G. Deppe, J. C. Campbell, R. Kuchibhotla, T. J. Rogers, B. G. Streetman, "Optically-coupled mirror-quantum well InGaAs--GaAs light emitting diode", Electron. Lett. 26 (20), 1665 (1990);
M. Ettenberg, M. G. Harvey, D. R. Patterson, "Linear, High-Speed, High-Power Strained Quantum-Well LED's", IEEE Photon. Technol. Lett. 4 (1), 27 (1992);
U.S. Pat. No. 5,089,860 Deppe, et al. Feb. 18, 1992, "Quantum well device with control of spontaneous photon emission, and method of manufacturing same";
EP-0550963 of Cho, et al "An Improved Light Emitting Diode".
The presence of a combination of critical parameters in the fabrication of Vertical Cavity Surface Emitting Lasers (VCSELs) makes such lasers suffer from nonuniformity effects over an epitaxially-grown wafer. Examples of parts of said VCSELs with critical parameter values are the two distributed Bragg Reflectors (DBRs), the cavity thickness and the thickness of the quantum well (i.e. the active region). This problem so far has limited array-production of operational VCSELs to 8*8 arrays. An 8*8 VCSEL array was disclosed in the publication "Fabrication of High-Packaging Density Vertical Cavity Surface-Emitting Laser Arrays Using Selective Oxidation" IEEE Phot. Techn. Lett. 8, 596 (1996), by Huffaker et al. Furthermore, the high current density needed for efficient operation of lasers (due to the threshold current needed for achieving inversion) limits the simultaneous operation of many laser elements in array applications. In addition, though VCSELs allow high-speed small-signal modulation, VCSELs cannot be used efficiently for high-speed large-signal modulation due to the presence of a threshold current.
The development of Microcavity light-emitting diodes (.mu. cavity LEDs) has created efficient and spectrally-narrow semiconductor light sources other than lasers in general and VCSELs in particular. In contrast to lasers, .mu. cavity LEDs do not suffer from any threshold behavior. State-of-the-art Microcavity LEDs have only one DBR, a wavelength cavity and one or more quantum wells that need to be matched in thickness, making design and production less critical. The absence of a threshold in Microcavity LEDs results in far lower current densities being required for array applications. The increased electrical to optical power efficiency of state-of-the-art .mu. cavity LEDs as compared to conventional LEDs improves the applicability of these .mu. cavity LEDs in applications such as optical interconnection systems (in particular as arrays of electromagnetic radiation-emitters) and display applications and systems that are critical on the power budget. State-of-the-art .mu. cavity LEDs use one or more quantum wells in the center of the cavity (see FIG. 1) that all have to be identical and matched to the cavity wavelength. Electrons and holes flow from opposite sides into the quantum wells and recombine. Switch-on and switch-off of the .mu. cavity LEDs are in essence radiative recombination time limited or RC time constant limited depending on which one is shorter. The use of several quantum wells has proven to be essential to reduce saturation in one quantum well, but moves the carrier localization away from the localization at the anti-node of the standing-wave pattern in the cavity. The use of several quantum wells also slows the response and the switch-on and switch-off of the .mu. cavity LEDs.
However, for array production of .mu. cavity LEDs, as for instance for optical interconnects, non-uniformities in the growth of these .mu. cavity LED and the signal modulation speed remain critical issues. In the design of .mu. Cavity LEDs one makes use of a quantum well to ensure carrier recombination at the center of the cavity standing-wave pattern (see H. De Neve, J. Blondelle, R. Baets, P. Demeester, P. Van Daele, and G. Borghs in IEEE Photon. Technol. Lett. 7, 287 (1995)). In the design of other state-of-the-art .mu. cavity LEDs such as disclosed by J. Blondelle, H. De Neve, P. Demeester, P. Van Daele, G. Borghs and R. Baets in El. Lett. 31, 1286 (1995) three quantum wells were used to boost efficiency. In said publication, the efficiency of a .mu. cavity LED is boosted by preventing saturation, at the cost of moving part of the active layer away from the localization at the anti-node of the standing-wave pattern in the cavity making the efficiency enhancement less pronounced. Thickness variations of the quantum well across a sample move the emission wavelength in the different area's of the sample away from the cavity wavelength which reduces the external efficiency. Furthermore, this device requires the thickness of the quantum well to be exactly matched to the wavelength of the cavity.
Ultra-high speed modulation in semiconductor-based electromagnetic radiation emitting devices in prior art publications so far was disclosed only for VCSELs or inefficient LEDs. T. J. de Lyon, J. M. Woodall, D. T. McInturff, R. J. S. Bates, J. A. Kash, P. D. Kirchner, and F. Cardone, in "Doping concentration dependence of radiance and optical modulation bandwidth in carbon-doped Ga.sub.0.1 In.sub.0.49 P/GaAs light-emitting diodes grown by gas source molecular beam epitaxy" Appl. Phys. Lett. 60 (3), 353-355 (1992) disclose a method of making high-speed LEDs by highly doping the active region of the LED, which leads to fast non-radiative recombination and hence a high-speed response of the LED. The resulting gain in speed however is more than compensated by a reduction in quantum efficiency which is incompatible with its use in arrays.
EP-0473983 discloses a light emitting device, the device concept of which uses cavity quantum electrodynamics. This device concept is based in the presence of a quantum well layer adjacent to a barrier layer. The device concept of EP-0473983 suffers a.o. from the problems of:
limited power performance; PA1 slow switch-off time; and PA1 emission wavelength shift during operation.